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Muter

Muter is a free, open-source utility for assessing Swift test suites through mutation testing.

Place
#12 of 25
Score
5.8
Muter's website

What Muter is

Muter is a free, open-source utility for assessing Swift test suites through mutation testing. It introduces code changes, called mutants, and reports a score based on how many the tests kill compared with the total introduced. Scores are available for the whole suite and individual files, with reports on surviving mutants. Muter runs within Xcode, from the command line or incrementally in CI. It supports Xcode projects and workspaces and Swift Package Manager, with output in plain text, JSON, HTML or Xcode issue navigator results. The documented operators replace relational operators, remove side effects, change logical connectors and swap ternary expressions. Muter works on a full copy of the codebase, leaving the original unchanged. It supports Swift projects tested with xcodebuild, including iOS, macOS, tvOS and watchOS projects, but the utility itself requires macOS 10.15 or later. It has no network access, collects no analytics and does not phone home. Mutation runs can take a long time, and some @resultBuilder mutations may fail to compile.

Good

  • Reports mutation scores for suites and files
  • Runs in Xcode, the command line and CI
  • Supports text, JSON, HTML and Xcode reports
  • Works on a copy of the codebase
  • No network access or analytics

Watch out

  • Supports Swift, not Objective-C
  • Runs only on macOS 10.15 or later
  • Mutation testing can take a long time
  • Some @resultBuilder mutations may fail to compile

Who it suits

Muter suits Swift developers using Xcode test targets or Swift Package Manager tests who want to assess how their test suite responds to code changes. The utility runs on macOS 10.15 or later.

Verdict

Muter provides several ways to run Swift mutation tests and inspect their results, including surviving mutants. Its scope is limited to Swift and macOS, and longer test runs need time to complete.

Plans and prices

From Muter's own pricing page, checked 8 Oct 2026.

All about pricing
PlanPriceWhat it gives you
Open-source MuterFreeNo paid plans or pricing listed

The facts

Each one read from Muter's own pages, with the date it was checked.

What it does

Compatibility caveat
Muter documents that mutations in @resultBuilder methods may fail to compile, with the limitation noted as potentially absent when using Swift 5.9 or later.checked 8 Oct 2026
Integrations
The README documents Xcode integration, Homebrew installation, and configuration support for Xcode projects and workspaces and Swift Package Manager.checked 8 Oct 2026
Language limit
Muter supports Swift code but does not currently support Objective-C.checked 8 Oct 2026
Mutation operators
The documented operators are RelationalOperatorReplacement, RemoveSideEffects, ChangeLogicalConnector, and SwapTernary.checked 8 Oct 2026
Mutation score
Each run reports a mutation score based on the mutants killed versus the total mutants introduced, with scores for the whole suite and individual files.checked 8 Oct 2026
Privacy
The README says Muter has no network access, collects no analytics, and never phones home.checked 8 Oct 2026
Purpose
Muter is a mutation testing utility for assessing the quality of Swift test suites.checked 8 Oct 2026
Reports
Muter can output reports as plain text, JSON, HTML, or Xcode issue navigator results.checked 8 Oct 2026
Run modes
Muter can run within Xcode, from the command line, and incrementally in CI.checked 8 Oct 2026
Runtime caveat
The README notes mutation testing can take a long time and recommends allocating enough time and considering separate schemes or targets for longer-running tests.checked 8 Oct 2026
Source handling
Muter performs its work on a complete copy of the codebase so it does not leave mutations in the original code.checked 8 Oct 2026
Test suites
It supports any kind of test target or suite when the application code is Swift, though the README recommends starting with unit tests because UI tests can take a long time.checked 8 Oct 2026

Where it runs

Supported app platforms
Muter says it supports projects tested with xcodebuild, including iOS, macOS, tvOS, and watchOS, and runs only on macOS 10.15 or higher.checked 8 Oct 2026

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